The present invention relates to a blood pressure calculation method for a non-invasive blood pressure measurement apparatus that measures a blood pressure by means of an oscillometric method. More particularly, the present invention relates to a blood pressure for a non-invasive blood pressure measurement apparatus configured such that a cuff being inflatable and deflatable and being fitted on a part of a subject's living body detects a change in an oscillation amplitude generated from arterial pulsation and a change in a cuff pressure, that a plurality of calculation methods is prepared as a calculation method for calculating a blood pressure value, for example, when a noise is generated due to body motion of the subject or when arrythmia due to disorder is observed or an abnormal pulse wave is generated with IABP (intra-aortic balloon pumping), and that a type of the noise or the arrythmia contained in the detection signal is determined and the optimum blood pressure calculation method is selected for each of the determined types of the noise or the arrythmia, thereby calculating and displaying the optimum blood pressure value.
In the past, in a non-invasive blood pressure measurement apparatus that measures a blood pressure by means of an oscillometric method, since a pressure noise resulting from subject's body motion is greatly superposed on the pulse wave of a cuff pressure, a predetermined logic element monitors the pulse wave of the cuff pressure to determine the presence of the noise resulting from the body motion. The non-invasive blood pressure measurement apparatus is provided with a means for generating a measurement error when it is determined that the noise is present, a means for adding information that represents occurrence of the noise during measurement, a means for reducing the noise resulting from the body motion, and the like.
In the related art, an electronic manometer is proposed that can measure a blood pressure with high precision even when a subject is walking, exercising or involved in other activities (see JP-A-2002-224059). The electronic manometer disclosed in JP-A-2002-224 059 includes: (1) a means for per forming frequency analysis on a pulse wave and a noise and transforming time series data into frequency data; (2) a means for comparing the frequency data of the pulse wave with the frequency data of the noise to calculate the frequency of the pulse wave; (3) a means for eliminating frequency components other than the frequency component of the pulse wave from the frequency data of the pulse wave; and (4) a means for transforming the frequency data of the pulse wave having other frequency components of the pulse wave eliminated therefrom into time series data to restore the pulse wave. Then, the electronic manometer calculates a blood pressure based on the restored pulse wave.
According to the electronic manometer disclosed in JP-A-2002-224059 having such a configuration, as a means for detecting a noise resulting from subject's body motion, a velocity sensor, an acceleration sensor, a position sensor, a displacement sensor, an angle sensor, a direction sensor, and an inclination sensor are used. In this case, when a photoelectronic sensor, for example, detects a photoplethysmograph signal and a noise resulting from subject's body motion is not present in the pulse wave signal, only a frequency component of the photoplethysmograph is present in the power spectrum of the photoplethysmograph, and no characteristic spectrum is present in the power spectrum of an acceleration signal. However, when the noise resulting from subject's body motion is present in the pulse wave signal, both a frequency component of the photoplethysmograph and a frequency component of the body motion are present in the power spectrum of the photoplethysmograph, and a spectrum of the frequency component of the body motion is present in the power spectrum of the acceleration signal. Therefore, the frequency component of the photoplethysmograph can be extracted by comparing the power spectrum of the photoplethysmograph with the power spectrum of the acceleration signal. That is, it is possible to remove the noise resulting from the body motion or the like from a pulse wave and calculate an appropriate blood pressure value based on the pulse wave.
Moreover, as a method for eliminating a noise resulting from subject's body motion or the like, contained in a pulse wave propagation signal detected from the subject, there are known (1) a method that compares a amplitude value (amplitude value) or an interval of pulse waves before and after a pulse wave signal of a cuff pressure and eliminates a waveform determined as being a noise; (2) a method that calculates moving average values of a plurality of varying amplitudes of a pulse wave signal of a cuff pressure; and (3) a method that eliminates a noise from a pulse wave signal of a cuff pressure by means of a mathematical method such as filter bank, FFT, or wavelet (see JP-A-2002-224061 and JP-A-2001-095766).
However, in the related-art blood pressure measurement method or blood pressure calculation method, when detecting the noise resulting from the body motion or the like to measure or calculate the blood pressure value, elimination of the noise or the determination as to invalidate or eliminate the measurement or calculation is performed by using one of the method described above. Therefore, the related-art method may not be the optimum calculation method depending on patients in various states and conditions, and it is thus difficult to perform the blood pressure measurement or calculation in a quick and appropriate manner.